Cane Toad Invasion in Australia

2/26/2021

Photo by Jaunathan Gagnon via Unsplash

The cane toad, exotically known as Rhinella marina, is a large, non-native amphibian introduced into Australia in 1935. Native to South and Central America, Giant toads are viewed as invasive species in Florida and Australia. They are poisonous to animals that try to devour them.

Not every non-native species introduced in a new region is invasive. They are labeled as invasive only when they compete with native flora and fauna for resources and alter or damage the ecosystem. Cane toads are invasive as they outnumber the native fauna with their breeding and insatiable appetite.

Cane Toad Color and Size

The warty amphibians come in a range of colors. They may be olive-brown, reddish-brown, grayish-brown with pale-yellow or beige-colored bellies and dark mottling. The adult cane toads are 15cm long on average (range from 6inches to 9inches in length).

Female cane toads are even more giant than male cane toads; the largest female measured was 24 cm long. Grown-up cane toads are larger than the native southern toads that only grow to 4 inches.

Cane Toad Adaptations and Diet

The cane toads are hefty, accommodating, and noxious throughout their life cycle. The desiccation-resistant toads can tolerate extreme temperatures around 40 degrees Celsius.

The fat, plump amphibians are carnivores at a young age but develop into omnivores as adults. They can eat anything that has a distant relation with food, be it vegetation, insects, lizards, smaller native toads, native frogs, snakes, little birds, and carrion of other mammals.

They are even attracted to food left in your backyard and scraps of human food on the table. Adult cane toads may even nosh on cane toad eggs. Living insects as beetles, honeybees, ants, and bugs are their favorite food. Because of this, they are creating food stress for insectivores within the ecosystem.

Image by G John via Pixabay

Cane Toad Habitat Range

The goliath toads are native to the United States, ranging from the southern United States to Central America and tropical South America. They are found in a wide range of habitats.

In Australia, their range extends from dunes to rainforests. They have profusely burgeoned because of the few native predators, therefore, they have less competition for food and space in yards, ditches, canals, and ponds.

Cane Toad introduction to Australia Beat Cane Beetles

About 100 cane toads were imported from Hawaii to north Queensland, Australia, in 1935 to control sugarcane beetles, pests to sugarcane crops. The larvae of the French’s Cane Beetle and the Greyback Cane Beetle damaged the roots of sugarcane. The beetles are native to Australia. They are harmful to sugarcane crops, which are a main source of revenue for Australia.

Adult cane beetles eat the crop’s leaves, and hatchlings feed on the roots. They have a large exoskeleton. Their eggs and larva are always laid to rest underground, making them difficult to destroy.

Those 100 cane toads that were released in a sugarcane plantation swelled in number to 3,000. Their adaptive and edacious nature that initially attracted farmers led them to conquer eastern and northern Queensland.

In 1964, they came across in the Gulf of Carpentaria. By 1978 they had entered New South Wales. In 1984 they had arrived at the Queensland/Northern Territory border. In March 2001, cane toads immigrated to Kakadu National Park, and by 2009 the horde is marching to the cooler areas near the western Australian border.

Image by S. Hermann & F. Richter via Pixabay

Both never crossed paths, for beetles don’t dwell near the ground, and cane toads are active at night. Completely forgetting about the beetles, cane toads diverted their attention to native insects. Insects are attracted to light at night, thus, they found ideal habitats under pipes, in ditches and ponds.

A failure at gobbling beetles, the species population has mushroomed considerably in numbers from the initial 3,000 cane toads to over 2 Million and counting.

In addition to Australia, they’re also found in south Florida. They were introduced in Florida in the 1930s. But an accidental release of 100 cane toads by a pet dealer in 1955 and 1960 has wrecked havoc, as reported by The Washington Post. Today, they are abundantly present in urban, suburban, and agricultural areas.

(EDDMAPS.2021. EARLY DETECTION & DISTRIBUTION MAPPING SYSTEM. THE UNIVERSITY OF GEORGIA-CENTER FOR INVASIVE SPECIES AND ECOSYSTEM HEALTH)

Cane Toad Reproduction

In Australia, the environmental impacts are not a big concern. The steadily sprouting populace, however, is. Cane toads are prolific breeders that can breed in the water of almost any nature and salinity level.

The mating season starts in September and ends in March. The species breeds year-round. An adult female can lay about 10,000 to 30,000 eggs at a time that hatch within 3 days. The tadpole stage lasts for about three to twenty weeks depending upon environmental conditions, they then metamorphose into toads and aggregate in numbers. Thousands of baby cane toads that emerge out of canals and pipes have made it to the news headlines:

“A mild winter and rain have led to a spring explosion of the amphibians, which are clogging pool filters, lawns, and driveways.”

Toads are sexually matured within two years. The average lifespan ranges from five years in the wild to 15 years in captivity. With thousands of cane toads emerging at the same time, there’s not an inch on the ground you can’t step on or drive without squishing a cane toad. They have clogged everything, from swimming pools, to gardens, sidewalks, and driveways.

Cane toads are openly loathed in Florida and Australia. Concerns have been raised about the safety of the domestic dogs that are their only fans. Dogs get addicted to licking the milky substance over the cane toads until they overdose.

Cane Toad Poisoning

An interesting factor that helped them increase is that they secrete a milky-white poison that kills local predators, even domestic pets that lick them.

The enlarged swellings, Parotid Glands, over the shoulder secrete the potent milky toxin known as Bufotoxin. Even their eggs and tadpoles are poisonous. The toxin is deadly enough to kill native predators, which is why cane toads secrete it as a defense.

Poison is loaded in the Parotid gland over the shoulder (Photo via Unsplash)

When cane toads sense a threat, they turn on their side with parotid glands towards the predator. They can even squirt a fine spray of poison. It is absorbed through mucous membranes and may cause skin and eye irritation to people who handle them.

The death of dogs from cane toad poisoning is common. About 50 dogs a year die in Hawaii after licking cane toads. In some countries, human deaths have also been reported after feasting on cane toads or consuming soup made from boiled cane toad eggs.

The poison acts primarily on the heart. The immediate symptoms after the ingestion of cane toad poison are hallucinogenic. These include salivation or frothing, twitching, vomiting, muscle spasm, convulsions, and cardiac arrest.

Cane Toad as Predator

The poison has made cane toads the predator to their own wild predators. The native fauna is not adapted to their poison. A 2002 study found a decline in Quoll (cane toad predator) population in Kakadu National Park, Australia, because it couldn’t adapt to the higher levels of Bufotoxin.

This poisonous toad has also resulted in the decline of goannas, estuarine Crocodile, Red Bellied Black Snake, and Western Quoll who die munching them. It has also affected the population of native insectivores by competing with them for food.

However, many species that can tolerate low levels of bufo-toxicity feed on cane toad’s tadpoles. These include dragonflies, water beetles, Saw-shelled Turtles, and Keelback Snakes.

Some predators have adapted to new methods of killing adult cane toads, such as native Torresian crows. The crow flips the cane toad on its back and delivers a blow to its throat, thus accessing the non-toxic innards.

Australian Freshwater Crocodile eat just its hind limbs. In toad-inhabited Australian regions, red-bellied black snakes have evolved to be resistant to toad toxin.

Image by sandid via Pixabay

Dealing with Cane Toad Invasion

The resilient cane toads have, without competition, conquered the top half of Australia. And the swathe of native fauna is due to the toad's self-defense. Significant efforts have been made to halt further cane toad migration. The spread was inevitable as the only way to eradicate them was by physically trapping the cane toads.

Many ideas have also been proposed to deal with the problem. These involve releasing sterile males into the population that would compete with other male toads but wouldn’t fertilize female eggs. Scientists can also genetically interfere with female toads to only produce male offspring. But these sound quite impractical.

Some suggest using poisonous cane toad sausages and playing on cane toad’s carnivore nature with hungry tadpoles cannibalizing the toad spawn.

The sausage approach is widely adopted by WWF in collaboration with Cane Toad Coalition in Kimberley, Western Australia. This would enhance the immune response to mild toxicity in large predators as the yellow-spotted monitor, freshwater crocodiles, northern quolls, northern blue tongue lizard, and sand goannas.

How to kill Cane Toad? Lungworm Parasite is the Answer.

Professor Rick Shine of the University of Sydney suggests using lungworm parasites to infect cane toad, which killed 30% of the laboratory baby toads.

“Lungworm parasite kills cane toads but doesn’t attack Australian frogs; a phenomenal opportunity for biological control,” says Professor Shine.

The team also identified the “alarm pheromones” released by the frightened cane toads to warn and communicate between tadpoles. These signals have a significant effect on the survival of the cane toads.

Intensified signals stress out the tadpoles leading to the death of about 50% of the population before adulthood. Those that metamorphoses were half the average size. Attractant pheromones are also helpful at luring and trapping cane toads. And luckily, they didn’t affect the native Australian frogs.

A combination of the cane sausages and alarm pheromones would be an effective strategy against cane toad invasion. Professor Shine is optimistic that with community efforts, the mighty cane toads can be defeated.

Author

Nida Riaz is a freelance blogger based in Pakistan. She started writing about her passion for the environment when the world came to a stop in early 2020.

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